To protect processing equipment, and in particular gas processing equipment, against unacceptable inflow of liquid, which also may contain sand and other particles, hereafter referred to by the collective term “sand”, a liquid separator is as a rule placed upstream of the equipment. Liquid and sand are thus collected such that gas and liquid, with sand, can then be treated separately.
Such protection of subsea compressors against too great an inflow of liquid and sand is previously known, and is generally effected by placing a liquid separator upstream of the compressor, such that liquid and sand can be separated from the well stream, collected and pumped into the gas transport pipe at a point downstream of the compressor, or optionally that the liquid is conveyed in a separate pipe.
Liquid separators may in this context mean, inter alia, separators, scrubbers, cyclones and liquid slug catchers, all of which, in addition to the actual separator, have a volume for collected liquid. This collecting volume will be determined by several factors such as:                Average liquid content of the well stream gas. This may vary enormously depending on whether the well stream gas comes from a dry gas field or a gas condensate field. There may be a field-dependent variation from 0.01% by weight or lower to 5% by weight or more, without this having any significance for the invention other than the practical dimensioning and operation. In multiphase pumping from the oil field, the liquid fraction may typically be 2% by volume to 30% by volume.        Liquid slug volume, i.e., the volume of a liquid accumulation which for various reasons has occurred in the pipe system upstream of the compressor, and which flows into the liquid separator in the course of a few seconds.        
To illustrate some of the disadvantages of the previously known solutions there follows a description of a common way of draining liquid from a subsea liquid separator with an associated volume for liquid collection. Reference is therefore made to FIG. 1 which illustrates the main equipment in such conventional subsea compression and pumping stations. Table 1 names the components to which the letters in the figure refer.
TABLE 1ALiquid separator with collecting volume in common vesselBCompressorb′Compressor motorCPumpc′Pump motorDLower permitted control level for liquidEUpper permitted control level for liquid when flow is stableFHighest liquid level, determined by liquid slug volumeGSecondary cleaning equipment, e.g., cyclonesg′Lower edge of secondary cleaning equipmentHDownpipe for liquid from secondary cleaning equipmentIOutlet from downpipeJAnti-surge valve with actuatorKAnti-surge coolerLCable for supply of electric power to compressor motorMCable for supply of electric power to pump motorNLiquid recirculation pipeOGas recirculation pipep, p′, p″, p″′Shut-off valvesQElectrical connector for compressor motorq′Electrical connector for pump motorRLiquid circulation valve
During normal operation all the illustrated shut-off valves, p, p″, p′″, are open and the anti-surge valve, j, is closed. At a given time, the compressor, b, runs at a certain speed in order to give desired gas production. The compressor is run by the electric motor, b′, which is supplied with electric power through the cable, l, that is connected to the compressor motor by an electrical connector, q. Similarly, the pump receives electric power through the cable, m, and the connector, q′.
The gas that flows out of the reservoir well, i.e., wet gas, to the liquid separator with its collecting volume, a, contains a certain average liquid content which under certain conditions may be disturbed by a transient liquid slug of high liquid content and short duration. It is important to be aware that during operation it is rare that several such liquid slugs come in rapid succession because the gas over a specific period has a given average liquid content.
In FIG. 1 a specific permitted liquid level, from d to f, is indicated in the liquid collector. When the pump is a centrifugal pump that is capable of forming bubbles, the lower level, d, is determined by the pump requiring a minimum head for the lower liquid level, d, in relation to the suction of the pump, c. The required head, “Net Positive Suction Head Required” (NPSHR) varies depending on the structure and operating conditions of the centrifugal pump, especially its speed, but may, for example to be of 3 to 4 meters. The lower liquid level, d, must also be so high that the centrifugal pump is protected against entrainment of free gas in its liquid stream. Centrifugal pumps are sensitive to free gas because the pumping capability, i.e., the capability to create pressure increase and capacity, diminishes together with the degree of efficiency, and the need for operating power increases. A common rule is that free gas in centrifugal pumps should be kept lower than 3% by volume. When the requirement for NPSHR is met, this rule is also observed automatically.
Furthermore, the highest permitted normal liquid level, e, when flow is stable is determined by the protection against unduly high amounts of liquid being entrained by the gas and passed into the compressor when the largest liquid slug, i.e., the dimensioning slug, comes on top of the upper permitted normal level, e, when flow is stable. The highest liquid level, f, is given in that the “largest liquid slug”—determined by calculations, measurements or empirically—is to have room on top of the upper normal liquid level, e, without the absolute upper permitted highest liquid level, f, being exceeded. It should be mentioned that the absolute highest liquid level, f, as regards location of the secondary cleaning equipment, g, when cyclones or other secondary cleaning equipment requiring downpipe, h, for draining is used, is determined by the drop in pressure across the secondary cleaning equipment, which is installed in the upper part of the liquid separator, a. The length of the downpipe, h, from the lower edge, g′, of the secondary cleaning equipment down to the highest permitted liquid level, f, must give sufficient static height to drain the secondary cleaning equipment which often consists of cyclones that have a drop in pressure in the range of 0.1 to 0.5 bar. Furthermore, the outlet, i, from the downpipe, h, must always be submerged in liquid to prevent gas from being sucked up through the downpipe, h. This means that the outlet, i, must be located below the lower permitted liquid level, d.
If simpler equipment, e.g., wire mesh mats, provide satisfactory secondary cleaning and thus droplet removal, the height between the secondary cleaning equipment, g, and the highest liquid level, f, can be reduced because the downpipe then becomes unnecessary. The mechanism for ensuring that liquid droplets that are caught in wire mesh mats and the like, is that the droplets fuse together to obtain a size that causes them to fall down through the gas rising towards the wire mesh mats, i.e., that the fall rate for the droplets is greater than the gas rate upwards.
What constitutes an “unduly high” liquid and sand load for the compressor depends on how robust its structure is in relation to this load, and the choice of materials and any protective coating against erosion on the compressor impellers. Centrifugal compressors can withstand an infrequent and transient high liquid load, e.g., 2% by volume, provided that the droplet diameter is not too large, i.e., typically less than 50 μm. Compressor suppliers also state that compressors can be run continuously with liquid, provided that the liquid content is less than 2% by volume. Other suppliers of centrifugal compressors state that compressors can be run with up to 2% by volume of liquid continuously in the inlet, droplets smaller than 50 μm, with acceptable erosion and lifetime.
During operation, the pump for the conventional solution is so controlled that the level in the liquid separator is kept between the upper liquid level, e, and the lower level, d. It is then usually controlled towards an “ideal level”, somewhere between d and e. This is a level that is determined to protect the pump against both bubble formation and entrainment of free gas, and which at the same time is sufficiently low to prevent liquid entrainment to the compressor.
The liquid that is separated out in the liquid separator, a, is collected in its collecting volume. In known solutions, the pump, c, is indicated as a centrifugal pump. These pumps are well suited for pumping when the liquid production in cubic meters per hour, m3/h, is not too small, so that the pumps can then be designed for the rise in pressure that may be required. Typically, the need for a rise in pressure can vary from 5 bar to 100 bar and even more.
As an example to illustrate the problems associated with known solutions, there may be chosen a typical case of a smallish gas field which only requires one compressor, and where the liquid production is 10 m3/day, i.e., 0.4 m3/h. In the example in question, this corresponds to a liquid content in the gas of about 0.01% by volume and a required rise in pressure of 30 bar from suction pressure which is 10 bar. There are no centrifugal pumps which, with continuous operation, can satisfy such a small requirement for volume flow with the necessary increase in pressure. One solution for continuous operation of the pump may involve recycling almost the whole volume of liquid so as to obtain satisfactory minimum liquid flow into the pump, e.g., 70 m3/h.
When comparing the liquid load that centrifugal processors can withstand in relation to the liquid content in fields of gas or a mixture of gas and condensate, as mentioned above, centrifugal compressors can in theory be run without liquid separation from the gas. However, this is a theoretical consideration which requires that the liquid should flow evenly dispersed in the gas. This state may be considered as normal for most of the operating time for a subsea compressor, but can sometimes be disturbed by larger liquid concentrations, in the worst case in the form of liquid slugs which fill the whole pipe cross-section. The mechanisms that result in the occurrence of such liquid slugs are typically changes, i.e., transients, which lead to liquid accumulation, e.g., at the start-up or shutdown of one or more wells on a template. The worst case is probably the start-up of the wells on the template where all the wells have been shut down. A great deal of liquid may then collect and flow towards the compressor. To avoid the liquid separator, a, having to be dimensioned to withstand the transient liquid slug at the start-Up, special start-up procedures may be devised. For example, the liquid slug can either be run past the compressor in a separate by-pass pipe or run in portions through the liquid separator, a.
Regardless of whether the compressor tolerates liquid, it is good protection against unnecessary wear or breakdown to conduct the liquid, which also has a certain sand content, around the compressor, especially when, as made possible by the present invention, a separate pump with power supply is not required.
For the compressor, it is thus its robustness against liquid and sand that determines the design of the gas processing part of the liquid separator, and similarly it is the robustness of the pump as regards bubble formation and entrained gas that determines the structure of the liquid processing part. As regards the setting of the accuracy and complexity of the level control, the same robustness of the two parts is also of particular importance.
FIG. 2 illustrates how use of a centrifugal pump pushes up the total constructional height of pump and liquid separator and its collecting volume in order to meet NPSHR.
It can be seen from the example that a height difference between the lowest liquid level and the intake to the pump is 4 meters.
To determine the total constructional height of the arrangement of compressor, liquid separator/collector and pump, it must be taken into account that the compressor and/or the compressor motor may require draining. In known solutions, gravity is used for draining. To ensure draining by gravity, the lower part of the compressor must be located approximately 0.5 meters above a lower level in the liquid collector.
The consequence of using a centrifugal pump and draining by gravity is a large constructional height for the whole arrangement as mentioned in the paragraph above. In FIG. 2 as an example it is indicated as 10.5 meters. A typical diameter of some components is also indicated.
In the example, a vertically oriented compressor and compressor motor are shown. If the two components are horizontal, the constructional height is reduced, but on the other hand the width increases.
FIG. 2 includes only components that are necessary to illustrate the need for height. The symbols here are the same as for FIG. 1, but in addition there is
TABLE 2zDrainage pipe for compressor with compressor motor